Time: All times of the year, but particularly during winter
and early spring when greenhouses are drier

Hosts: over 300 woody and herbaceous species

Twospotted spider mites can occur at any time of the year in the
greenhouse; however, populations are more severe in dry conditions,
particularly in winter and early spring. They attack over 300 host
plants; both woody and herbaceous plants are at risk.

Adults are 0.4 mm long and have eight legs. They are easily
recognized by the two to four black spots on the semi-transparent
body. Eggs are round and transparent. Larvae have six legs, and nymphs
have eight.

Twospotted spider mites damage plants by inserting their chelicerae into
plant tissue and sucking the contents of leaf cells. This causes
characteristic stippling on the leaves. In heavy infestations, the mites
produce extensive webbing over plants and can cause leaf drop. Monitor
for mites by looking for damage and examining the undersides of leaves
for mites and eggs. Beat plants with a stick and collect mites and
eggs over a white cloth. Mites mature in 5 to 20 days depending on
temperature and females lay over 100 eggs during their lives.

Management: Small populations of twospotted spider
mites can be managed with oils, soaps, or biological control agents.
Washing leaves with water may dislodge some of the mites, and increasing
the humidity in the greenhouse will make a less favorable habitat. In
heavy infestations, miticides should be used. Rotate between classes of
miticides to prevent spider mites from becoming resistant to the
miticides.

Darkwinged fungus gnats are small flies
that commonly occur in the greenhouse. Larvae feed on roots of plants and
mature in two weeks. They are white with black head capsules and reach
5.5 mm in length. Adults are small (2.5 mm long), dark gray to
black, with long legs and long, filiform antennae. They live for one
week and females can lay up to 150 eggs on the soil surface during
that time. Adults are weak fliers and do not feed.

Fungus gnats are usually considered
nuisance pests, but larvae sometimes vector root rot pathogens to plants.
Monitor for larvae by placing potato slices on the surface of the soil,
and monitor for adults with yellow sticky cards. Wilted plants may
indicate the presence of fungus gnat larvae and root rot.

Thrips feeds on the flowers and foliage by inserting its
modified left mandible into the tissue, and sucking the fluids from cells.
Oviposition and feeding scars reduce the aesthetic quality and
marketability of ornamental plants. These thrips spread tomato spotted
wilt (TSWV) and impatiens necrotic spot viruses (INSV).

Females lay eggs in tender plant tissue. The eggs hatch in 2
to 14 days, depending on temperature. Larvae are found in the protection
of perianth of the flower or within developing terminal foliage.
Late in the second instar they stop feeding and move down the plant
to pupate. Thrips develop through two quiescent, non-feeding pupal
stages in the soil, plant litter or in a protected area on the plant.
Adults emerge and resume feeding on flowers, buds, and terminal
foliage. The entire life cycle from oviposition to adult emergence can
take 12 days in hot weather to 44 days in cool weather.

Management: Rotate classes of
insecticides to prevent resistance. Use blue or yellow sticky cards to
monitor for adults.

Thrips feeds on the flowers and foliage by inserting its
modified left mandible into the tissue, and sucking the fluids from cells.
Oviposition and feeding scars reduce the aesthetic quality and
marketability of ornamental plants. Greenhouse thrips move relatively
slowly and rarely fly. They prefer a cool, shady, and fairly moist
atmosphere. Reproduction is usually by parthenogenesis (laying
unfertilized eggs). Under optimum conditions the time for development
is 17 to 20 days for the eggs, about 13 days for the two larval
instars, and about 5 days for the prepupal and pupal stages. The
adults can live 7 weeks on plants growing in the greenhouse. Dark
spots of excrement are often noticeable on the leaves and fruit. Pupae are found on leaves.

Management: See western flower thrips.

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